Roiling, incandescent magma and boiling gases covered the earth in the wake of its formation 4.6 billion years ago. Regions of this fiery sea eventually cooled enough to crust over, leaving the planet’s first hard rocks floating like slag on the white-hot liquid. But they were nothing more than a thin veneer. The thick roots of terra firma were much longer in the making.

Exactly how—and how quickly—continents arose and grew is a matter of ongoing debate. Scientific wisdom long held that the earth’s inner workings alone drove continent formation. But recent findings have turned the spotlight toward a once heretical idea: that large asteroid impacts played a constructive role as well.